Add problem 235 - Lowest Common Ancestor Of A Binary Search Tree

Author: anirudhology

problems/tree/lowest_common_ancestor_of_a_binary_search_tree.py

@@ -0,0 +1,20 @@
+from problems.util.tree_node import TreeNode
+
+
+class LowestCommonAncestorOfABinarySearchTree:
+    def lowestCommonAncestor(self, root: 'TreeNode', p: 'TreeNode', q: 'TreeNode') -> TreeNode | None:
+        # Special case
+        if root is None or p is None or q is None:
+            return None
+        if root.val > p.val and root.val > q.val:
+            # If value of root is greater than values at both p and q,
+            # it means LCA can only be in the left subtree
+            return self.lowestCommonAncestor(root.left, p, q)
+        elif root.val < p.val and root.val < q.val:
+            # If value of root is smaller than values at both p and q,
+            # it means LCA can only be in the right subtree
+            return self.lowestCommonAncestor(root.right, p, q)
+        else:
+            # We will reach here if value is root is in between values at
+            # p and q. If this happens, then only root can be the LCA
+            return root

tests/tree/lowest_common_ancestor_of_a_binary_search_tree_test.py

@@ -0,0 +1,52 @@
+import unittest
+
+from problems.tree.lowest_common_ancestor_of_a_binary_search_tree import LowestCommonAncestorOfABinarySearchTree
+from problems.util.tree_node import TreeNode
+
+
+class LowestCommonAncestorOfABinarySearchTreeTest(unittest.TestCase):
+    # noinspection PyTypeChecker
+    def test_lowest_common_ancestor(self):
+        lowest_common_ancestor_of_a_binary_search_tree = LowestCommonAncestorOfABinarySearchTree()
+
+        # Test case 1: Both nodes are null
+        self.assertIsNone(lowest_common_ancestor_of_a_binary_search_tree.lowestCommonAncestor(None, None, None))
+
+        # Test case 2: One node is null
+        root = TreeNode(3)
+        p = TreeNode(1)
+        self.assertIsNone(lowest_common_ancestor_of_a_binary_search_tree.lowestCommonAncestor(root, None, p))
+        self.assertIsNone(lowest_common_ancestor_of_a_binary_search_tree.lowestCommonAncestor(root, p, None))
+
+        # Test case 3: Both nodes are the same
+        self.assertEqual(lowest_common_ancestor_of_a_binary_search_tree.lowestCommonAncestor(p, p, p), p)
+
+        # Test case 4: LCA is root
+        q = TreeNode(5)
+        root.left = p
+        root.right = q
+        self.assertEqual(lowest_common_ancestor_of_a_binary_search_tree.lowestCommonAncestor(root, p, q), root)
+
+        # Test case 5: LCA is in the left subtree
+        p2 = TreeNode(0)
+        p.left = p2
+        self.assertEqual(lowest_common_ancestor_of_a_binary_search_tree.lowestCommonAncestor(root, p2, p), p)
+
+        # Test case 6: LCA is in the right subtree
+        q2 = TreeNode(6)
+        q.right = q2
+        self.assertEqual(lowest_common_ancestor_of_a_binary_search_tree.lowestCommonAncestor(root, q, q2), q)
+
+        # Test case 7: Complex tree
+        root2 = TreeNode(6)
+        p3 = TreeNode(2)
+        q3 = TreeNode(8)
+        root2.left = TreeNode(0)
+        root2.right = TreeNode(4)
+        root2.left.left = TreeNode(3)
+        root2.left.right = TreeNode(5)
+        self.assertEqual(lowest_common_ancestor_of_a_binary_search_tree.lowestCommonAncestor(root2, p3, q3), root2)
+
+
+if __name__ == '__main__':
+    unittest.main()